Radio direction finder



Dec. 2, 1952 E. D. BLODGETT ETAL 2,620,471

RADIO DIRECTION FINDER Filed March 28, 1945 2 SHEETS-SHEET 1 W r IWWDec. 2, 1952 E. D. BLODGETT ETAL 2,620,471

RADIO DIRECTION FINDER Filed March 28, 1945 (1 i g/ Fifi/4f)? 2;

Patented Dec. 2, 1952 FFEQ RADIO DIRECTION FINDER of DelawareApplication March 28, 1945, Serial No. 585,246

Claims.

This invention relates to antenna systems, and more particularly toimprovements in directional antennas for radio direction finders and thelike.

The principal object of the present invention is to provide improvedantennas of the spaced element type of greater angular sensitivity thanis achieved by prior art systems.

Another object is to provide antennas of the described type havinggreater discrimination against signals of undesired polarization thanthose commonly used.

A further object is to provide antennas of the described type capable ofefficient operation through a relatively wide band of frequencies.

The above and other objects will become apparent to those skilled in theart upon consideration of the following description with reference tothe accompanying drawings, of which Figure 1 is a schematic elevation ofa prior art antenna system of the so-called elevated H Adcock type,

Figure 2 is a polar graph of the usual horizontal directive patternobtained with antennas of the type shown in Figure 1,

Figures 3 and 4 are polar graphs of the horizontal directive patternsobtained with the antenna of Figure 1 with spacings between the elementsof one wavelength and two wavelengths respectively,

Figure 5 is a schematic plan view of an antenna comprising a singledipole and a reflector,

Figure 6 is a polar graph of the horizontal directive pattern of theantenna of Figure 5,

Figure '7 is a schematic view of an antenna system comprising twoantennas like that of Figure 5, directed oppositely to each other,

Figure 8 is a polar graph of the horizontal directive pattern of thesystem of Figure 7,

Figure 9 is a schematic plan view of a system similar to that of Figure7 but modified to provide differential action between the two elementsin accordance with the present invention,

Figure 10 is a polar graph of the field patterns of the individualantennas of the system of Figure 9,

Figure 11 is a polar graph of a horizontal field pattern obtained in theoperation of the system of Figure 9,

Figure 12 is a schematic diagram of a modificaation of the system ofFigure 9, including a sense antenna and means for periodically varyingthe directive pattern of the array,

Figure 13 is a polar graph illustrating the horizontal field patterns ofthe elements of the system of Figure 12, and how the patterns arecomput.

bined with one polarity relation between the directive antennas andthesense antenna,

Figure 14 is a polar graph similar to that of Figure 13 but showing theresultant field patterns with the opposite polarity relation between thedirective antennas and the sense antenna,

Figure 15 is a perspective view of an antenna like that shownschematically in Figure 9,

Figure 16 is a schematic diagram of a system like that of Figure 9, butwith the elements connected additively rather than differentially, and

Figure 17 is a polar graph showing the directive pattern of the systemof Figure 16.

An important requirement of an antenna to be used for direction findingis that it must discrimi nate against signals which are polarized in amanner other than that to which the system is intended to respond. Inother words an antenna designed to operate with vertically polarizedsignals must exhibit as little response as possible to horizontallypolarized energy. The more fully this requirement is met, the less thepossibility will be erroneous bearing indications on downcoming signals,such as those reflected from the ionosphere or transmitted from a highaltitude.

One well known type of antenna which provides relatively little responseto signals of undesired polarization is the elevated H Adcock,illustrated in Figure 1. This antenna comprises a pair of verticaldipoles l and I, spaced horizontally from each other a distance d andsupported at an elevation it above the earths surface. The dipoles I andI are connected together difierentially by conductors 3 and 3', and to aradio receiver (not shown).

The elevation h is made as great as possible to avoid unbalance betweenthe upper and lower dipole elements owing to capacitance to ground. Thespacing d is ordinarily made appreciably less than one-half wavelengthat the highest frequency at which the system is to be used. Referring toFigure 2, the line A-A represents the common vertical plane of theantennas l and I. A signal arriving from any point on the line OC,perpendicular to AA and equidistant from the antennas, will induce equalvoltages, in phase with each other, in the two dipoles. Due to thediflerential connection between the antennas, as shown in Figure 1, thetwo voltages will cancel each other, producing zero resultant output.

A signal arriving from any point on the line AA on the other hand, willinduce voltages of different phases in the two antennas, whichvectorially subtracted will provide a maximum out- Similarly, at anyangle 0 to the line 00,

the output is a function of the direction of arrival of the signal. Thepolar diagram of the horizontal directive pattern is the well knownfigure-of-eight curve, having two maxima in opposite directions alongthe line A-A and two minima along the line C.

The maxima are quite broad, as is evident from Figure 2, and for thisreason are not generally used for direction finding. The minima are muchnarrower. The direction of signal arrival may be determined. by rotatingthe array until minimum or null signal is received. The line O-C willthen indicate the direction.

The sharpness of the null may be increased by increasing the spacing 11,so that the phase difference between the voltages induced in the twoantennas is greater for a given angle 0 between the plane of theantennas and the wave front of the signal. When this is done, however,it is found that the pattern breaks up into more lobes, giving fourminima and four maxima with a spacing of one wavelength, as shown inFigure 3, and eight lobes, as shown in Figure 4, with a spacing of twowavelengths. Thus, although the null points are much sharper, theambiguity of three or more undesired nulls is substituted for the singlereciprocal null shown in Figure 2. Furthermore, while the reciprocalnull of Figure 2 may be determined by use of an auxiliary sense antennain the well known manner, application of this method to patterns of thetypes of Figures 3 and 4 will only reduce the ambiguity by one-half,leaving at least two nulls which cannot be differentiated from oneanother.

The present invention contemplates the use .of reflectors with thespaced antenna elements, so arranged as to prevent the appearance ofundesired nulls. Referring to Figure 5, a vertical antenna element 5 issupported parallel to and a distance s in front of a fiat reflector l.The horizontal directive pattern, in polar coordinates,

rcos sin 0) where 1' is the relative response at an angle 0 to the line00 perpendicular to the plane of the reflector l, and A is thewavelength. This p ttern is shown in Figure 6 for s=)\/4.

If two structures like that of Figure 5 are placed back-to-back, asshown in Figure 7, and connected either differentially, as shown, oradditively, to a common utilization circuit, the directive pattern willbe a figure-of-eight (see Figure 8) consisting of two lobes similar tothe pattern of Figure 6. It is to be noted that the pattern shown inFigure 8 does not result from differential action between the antennas,as does the pattern of Figure 2, but is merely the sum of twoindependent patterns. Consequently, the spacing d in the system ofFigure I has no effect on the shape of the pattern. The pattern is alsoindependent of any phase difference introduced in the connecting meansbetween the two antennas. The reason for this is that only one or theother of the antennas responds to a signal from a particular direction,no matter what that direction is.

Referring to Figure 9, the two reflectors l are placed at an angle awith respect to each other, with the antennas 5 outside. This causes thecorresponding directive lobes to be at the angle a with respect to eachother, as shown in Figure 10. It is evident that the lobes now intersectthrough their portions which lie within the angle a. A signal arrivingalong the line 0C, lying within the angle a, will induce a voltage ineach antenna. A signal arriving along a line OP, outside the angle a,will act only on one antenna. Within the opposite angle a, neitherantenna will respond. With the antennas differentially connected, asshown in Figure 9, a signal arriving from any direction within theanglea will produce a differential output which is zero at the intersectionof the two lobes. The resulting directional pattern of the array ofFigure 9 is shown in Figure 11.

The pattern of Figure 11 actually has two nulls, one from the forwarddirection and the other in the region 0.. However there is no ambiguitybecause the back null extends over the entire angle a, while the forwardnull is very sharp. There is no definite limit to the spacing which maybe used between the two antennas. If the distance is made such as toproduce a large number of lobes without the reflectors, the angle a.should be adjusted to include only part of the two desired lobes. Thesharpness of the resulting null is approximately that of the multiplelobed pattern.

Under some conditions it is desirable to provide an indication of thedirection in which the antenna array should be rotated to'bring the nullupon a signal being received. This may be done by means of an auxiliaryantenna, connected like the sense antenna of the conventional L-Rindicator type of radio compass. Referring to Figure 12, an auxiliaryantenna :9 is provided between the directive antennas 5. This antennamay also be provided with a reflector H. The antennas 5 aredifferentially connected, as .in the system of Figure 9 to a commonutilization circuit, in this case a radio receiver 13. The antenna 9 iscoupled to the receiver l3 through a phase shifter l5 and a reversingswitch IT. The output circuit of the receiver is applied through asecond reversing switch l9 to a meter 2|. The switches l1 and I9 arearranged to operate together, as indicated by the dash line connectiontherebetween, and may be operated periodically by a motor 23.

Referring to Figure 13, the directive pattern of the antenna 9 is asingle lobe 25 of the type shown in Figure 6. The pattern of theantennas 5 comprises the two lobes 21 and 29.. Since the antennas 5 aredifferentially connected, the lobes 21 and 29 are of oppositepolarities, i. e. a signal received in the lobe 2'! will produce avoltage at the receiver [3 out of phase with the voltage the same signalwould produce if received in the lobe 29. The voltage induced in thesense antenna 9 is 90 out of phase with the vector difference of thevoltages induced in the two antennas 5. After passing through the phaseshifter I5, the sense voltage will be in phase with that of one of theantennas 5, for example the one on the right, and 180 out of phase withthat of the other antenna 5. This voltage is added in the receiver 13 tothe combined output of the antennas 5, providing a resultant directivepattern which is unsymmetrical as shown by the dash line curve 3| ofFigure 13. The null of the pattern represented by the curve 35 isdisplaced to the left of the null of the antennas 5, by an angl 5.

Now if the connection of the antenna 9 to the receiver 13 is reversed,the output of the sense antenna will add to the left hand lobe andsubtract from the right hand lobe providing the pattern shown by thecurve 33 in Figure 14. The null of this pattern is displaced by an angle18 to the right of the center line. The curve 3| is reproduced in Figure14 for comparison with the curve 33.

The output of the receiver I3 is applied to the meter 2| through theswitch I9. When the pattern 33 appears, the meter is energized fordeflection to the left, and when the pattern 3| is present, the meter isenergized for deflection to the right. The switches I! and I9 areoperated at a sufliciently rapid rate for the inertia of the meter (andthe electrical inertia of its associated circuits) to cause integrationof the receiver output pulses corresponding to the signals alternatelypicked up with the patterns 3| and 33. Thus if the received signalarrives along the line O-C (Figure 14) the successive pulses of receiveroutput are equal and the meter 2|.reads zero. If the signal arrivesalong a'line to the left of O-C, the pulses corresponding to theconnection for the pattern 33 are greater, and the meter 2| is deflectedtoward the left. Similarly, a signal arriving from the right of the line-0 causes the meter 2| to deflect to the right.

The arrays shown schematically in Figures 9 and 12 are not necessarilylimited to one antenna element for each reflector. In fact it has beenfound preferable in practice to use two or more horizontally spacedelements with each reflector, with the elements of each group(associated with one reflector) interconnected to provide a singlelobedirective pattern similar to that of Figure 9, but narrower.

Referring to Figure 15, a pair of reflectors 7 are mounted by metalstraps 35 upon a vertical mast 3?. The reflectors i are disposed invertical planes at an angle a. with respect to each other, as describedabove. The reflectors 7 comprise rectangular networks of metal tubing39, supporting and connecting together a large number of vertical wires4|. A pair of vertical dipoles and 5' are supported on blocks 43 ofinsulation at the ends of tubular members 45, which are in turn securedto plates 4? fastened to the reflector.

Transmission lines 49 are connected to the respective elements of thedipoles 5 and 5', and extend through the tubular members 45 to a linebalance converter 5| behind the reflector 1. The converter 5| is notshown in detail, but may be of any known type, such as that described onpage 855 of Radio Engineers Handbook, by F. E. Terman, published in 1943by McGr-aw Hill Book Company. Th upper element of the dipole 5 isconnected to the same balanced terminal of the convertor 5! as the upperelement of the dipole 5', and the lower elements of the dipoles 5 and 5'are connected together to the other balanced terminal of the convertor5|. The connections of the dipoles 5 and 5' to the convertor 5| of theleft hand array are opposite to the corresponding connections of theright hand array. The unbalanced output lines 53 of the convertors 5|are connected together near the mast 31, and to a main downlead line 55.

The horizontal spacing between the elements 5 and 5 of each pair ispreferably somewhat more than one-half wavelength at the mean frequencyof operation. The dipoles are spaced approximately one-quarterwavelength from the reflectors 1, and are about one-half wavelength longover all at mid-band. The elements are of large diameter (approximatelyA wave length) to provide broad resonance characteristics in order tominimize variations of antenna impedance with frequency. The linebalance convertor 5| is preferably of the reactance-compensated type.

Although the invention has been described as embodied in radio directionfinder systems, it will be evident to those skilled in the art that thedescribed antennas'may be used with advantages for other purposes suchas directive transmission, for example. For transmission it is foundpreferable to connect the spaced elements additively rather thandifferentially, as shown in Figure 16. This provides directive patternsof the type shown in Figure 1'7. The beam is sharper than that whichwould result if the elements 5, 5 were not shielded from each other bythe screens 1.

Summarizing briefly, the invention has been described as an improvedtype of directive antenna system including spaced differentiallyconnected elements, with reflectors arranged so that the directivepattern of one group intersects but does not coincide with that of theother, oppositely connected group. Thus each group alone respondsthroughout a respective wide azimuth angle, and both groups responddifferentially throughout a common narrow azimuth angle. Thisarrangement provides extremely sharp directivity without theintroduction of undesired directive lobes.

We claim as our invention:

1. In a radio direction finder operable over a wide band of frequencies,a pair of rectangular planar reflector members pivotally joined togetherat adjacent edges, at least one dipole antenna element mounted upon andlocated centrally in front of each of said planar reflector members,voltage amplitude responsive indicator means, and means to couple saiddipole antenna elements to said indicator means in opposition to eachother, said reflector members being angularly adjusted to substantiallyshield said dipole antenna elements one from the other and to cause thedirective field patterns of the directive antennas formed by saidreflector members and the dipole antenna elements mounted thereon tooverlap.

2. A radio dirction finder operable over a wide band of frequencies,including voltage amplitude responsive indicator means, a pair ofsimultaneously effective directive antennas including plane reflectormembers arranged behind each of said antennas, said antennas andassociated reflectors being arranged at an angle with respect to eachother at which the directive axes of said antennas diverge appreciablyand the directive field patterns thereof overlap over a relatively smallangle only, said antennas being arranged at distances from saidreflectors at which the latter substantially shield the antennas onefrom the other, whereby the spacing between antenna elements hassubstantially no effect on the resultant field patterns, and meanscoupling said antennas in opposition to said indicator means, a senseantenna arranged between said reflectors and substantially shielded fromsaid directive antennas, means coupling said sense antenna to saidindicator in phasequadrature with respect to said directive antennas,and periodic switch means connected cyclically to reverse theconnections of said directive antenhas with respect to those of saidsense antenna.

3. A radio direction finder operable over a wide band of frequencies,including voltage amplitude responsive indicator means; a pair ofsimultaneously effective directive antennas inclucling' plan'ereflector; members sa'r'ranged behind each of said antennas,;,sa id'antennas an'd'associated reflectors" eifrgarran'gedatian iangle'atother; wherebyjthe spacing between antenna element'sihastsubstantiallyno efiect on'th'e result- Jant' field patterns,z:fand means couplingsaid antennas inoppositioni'tosaid indic'at'ormea'n's,

a sense antenna located between said reflector s -to;be. substantiallyshielded; from s'aid directiveantennas, means eeu nng said 'senseantennato said indicator 'means in phase quadrature with respect 'to said"directive antennas'and 'periodic switch. "means connected cyclically to,'reverse the connections of said direction anten-I' nas with respect tothose of said'sense antenna;

4. In a radio=direction 'fin'denoperablej ver -l a wide band of.frequencies, atl'eastthree simulr taneously I effective antenna elementsarranged substantially parallel to e'ach-cther on a; given line; twoplane reflector me'mbe'rsarranged be' hind said antenna elements, saidplane reflector members being arranged' at' angles. with respect to eachother at which" the axes of the directive antennas formed by'. theoutermost antenna 'elements and the adjacent plane reflecto'ri membersdiverge appreciably: and the directive field at, terns thereof overlapover a relatit'ely small angle only, the foremost edges of said" planereflector members being located-to substantially shield the antennaelement's one from the other Without substantially reducingthe-effectiven'e'ss of the innermost antenna 5 element, whereby thespacing between antenna elements has. substantiallyno effect on-theresultant".di-. rectional: field patterns, amplitude responsive-in ldicator' means, means coupling said outermost antenna elements inopposition to said indicator meansg-means coupling the innermostante'nna element to said indicator'means'in phase 'q'uadrature withrespect to said outermost antennas,

and periodic switch means connected cyclically to" reverse: theconnections or saidouterm'ost antenna elements with respect to those ofsaid innermost antenna;

-5'. -I-naradio "direction finder operable over i a wide band offrequencies, at least three antenwna elements arranged substantiallyparallel to each other: on a given line, two plane reflector membersarranged behind said I antenna elementsysaid planegrefiector membersbeing arimmah,

Number Name 7 .Date ,1 I 1,435,941 Robinson 1; Nov, 21,1922 -,l,491,372Alexanderson Apr. "22', 1924 I 1,724,246 W'rig'ht et a1. Augyl3,

, 1,898,831 Hahnemann 'Feb. 21, 1933 1,912,234 Willoughby -rMay 30, 1933r "2,017,909 Leib Oct.' .22, 1935 2,054,160 'Leib. Sept. 15, 19362,'145,876 Hinman, Jr. Feb. 7, 1939 2,187,618 Gerhard 1' Jan. 16,19402,216,708 'K0lster Oct. 1, 1940 2,217,321 'Run'ge et al. L Oct. 8, 1940-2,234,587- Budenbom Mar. 11, 194i 2,235,055 Von Ottenthal Mar. 18, 194112,286,804 Hooven June'16,"194 2 2,314,234 Meier' Mar. 16, 19432,418,308 Luck Apr-:1, 1947 2,419,994 Hansell May 6, 1947 2,448,006Starr Aug; 24, 1948 1 F N PATENTS Number 1 Country Date 441,964 GreatBritain Jan. 30, 1936 r'an'ge'd' tat:- an'gleslwithi respect to. eachIother at which. the axes of the.,directive antennas formed by theoutermost antenna 'elementsandf the ad jacent plane reflector membersdiverge appreciamy anditheldirective' field patterns thereofnover- 4 lapover a relatively smallangle only, thejforee most edges of said' planereflectormembers .Ibein'g located to substantially shield the. antennaEDWARD D. 'BLODGETT. LOUIS L LAKA'IO'S;

' REFERENCES CITED The following references are of record in the file ofthis patent: v

UNITED STATES PATENTS OTHER REFERENCES The Corner Reflector Antenna, byJohn D. Kraus, Proc. IRE, vol. 28,, No, 1, November .1940,

pages 513-519.

